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Description

MBC5 Study Guide Chapter 10 (Membrane Structure) All living cells have a boundary called the plasma membrane that separates the cytoplasm from the external environment. Eukaryotic cells also have internal membranes that create organelles such as the cell nucleus. In this chapter, we will focus on structural aspects of membranes and membrane proteins. Later chapters will consider functional issues. THE LIPID BILAYER When exposed to water, certain types of lipids form a bilayer (Figure 101). Membrane proteins may be embedded in the bilayer structure. In this section, we will consider structural features of lipids that cause them to form a bilayer, and we will examine the dynamic aspects of lipid bilayers and biological membranes. Phosphoglycerides, Sphingolipids, and Sterols are the Major Lipids in Cell Membranes The most abundant membrane lipids are phospholipids, and the most abundant phospholipids are phosphoglycerides (Figure 102). Phosphoglycerides are amphipathic because they have a polar head and two nonpolar tail regions. The backbone of phosphoglycerides is the three carbon compound glycerol, whereas the backbone of the phospholipid sphingomyelin is sphingosine. Many membranes contain cholesterol, a sterol that contains a rigid ring structure attached to a single polar hydroxyl group (Figure 104). Phospholipids Spontaneously Form Bilayers The nonpolar tails of phospholipids do not favorably interact with water (Figure 107). This unfavorable interaction decreases entropy, because water forms an ordered structure around the nonpolar tail in an attempt to minimize interactions. To avoid a decrease in entropy, which is energetically unfavorable, phospholipids in water will spontaneously form a bilayer in which the nonpolar tails interact with each other and polar heads are exposed to water. This minimizes entropy because water is more disordered. The Lipid Bilayer Is a TwoDimensional Fluid When exposed to water, phospholipids form a spherical structure called a liposome (Figures 10 8. 109). The bilayer is semifluid, allowing certain types of movements and preventing others o (Figure 1011). Lipids can diffuse laterally (e.g., from left to right) and rotationally (rotate 360). However, it is energetically unfavorable for them to flip from one leaflet of the bilayer to the other. The Fluidity of a Lipid Bilayer Depends on Its Composition An optimal fluidity of a biological membrane is needed to ensure the proper integrity of the membrane and to promote the function of membrane proteins. Fluidity can be altered by altering the lipid composition. As noted in Table 101, certain categories of phospholipids are common. You should be aware that structural features of lipids make the bilayer more or less fluid. These include the following: Double bonds in the lipid tails make the bilayer more fluid. This is because double bonds diminish packing between the tails. Shorter lipids tails make the bilayer more fluid. Shorter tails are freer to diffuse laterally and rotationally.